The present invention relates to an aqueous electro-dipping varnish/coating material, which can be deposited at the cathode and contains a cationic synthetic resin binder which can be made water-soluble by protonizing with acids, and a nonionic synthetic resin binder, and its use for coating electrical conductors which are operated as a cathode. The invention moreover relates to a process for coating electrical conductors using this coating material.
Electrical conductors, especially so-called winding wires, are coated primarily with baking varnishes, which contain organic solvents, binders and varnish additives, including catalysts for cross linking the binder. The electrical conductors are coated with these varnishes and then heated to temperatures above about 200.degree. C. in order to bake the varnish with cross linking. The coating and baking is repeated several times, in order to achieve an adequate coating thickness. Particularly great industrial importance has been achieved in this field by the polyesterimide resins (see British Pat. Nos. 937,377, 1,082,181, 1,067,541, 1,067,542; German Auslegeschrift 1,520,061, 1,520,068, 1,494,452, 1,494,454, 1,494,457, 1,937,311, 1,937,312, 1,957,157, 1,645,345, etc.).
The coating films of the well-known polyesterimide resins, which are baked onto the electrical conductors, especially copper wire, have excellent thermal stability, good elasticity, good solvent resistance and good behavior towards thermal shock. Polyesteramidimides are also of importance (see the above-mentioned German Auslegeschrift No. 1,494,457). Polyhydantoins are also known as wire coating resins (German Auslegeschrift No. 1,814,497).
In order to avoid the use of solvents, such as cresol, which are injurious to health and which, as a rule, are used in the known baking varnishes, the attempt was made to deposit water-dilutable wire coating resins anodically. However, because of the high resistance of the deposited films, only thin coatings were obtained. In the German Auslegeschriften Nos. 2,111,584 and 2,111,640, the logical proposal is therefore made to eliminate this defect by the addition of low molecular acids or by a lesser degree of neutralization. At the same time, polyesterimides or polyamidimides are used as binders, which have a high number of free carboxyl groups distributed in the molecular chain of the resin. This chemical structure leads to an electrochemical dissolution at the surface of the metallic workpiece, that is, a copper wire assumes a gray to green discoloration.
In the German Auslegeschrift No. 2,106,762, a sufficient film thickness is obtained by depositing consecutively, by the electro-dipping process, first a resin dispersion at a low voltage and then a water-dilutable varnish with a high film resistance at a higher voltage. The difficulty with this process lies in the fact that, in order to prepare the resin dispersion, emulsifiers must be added which remain in the film, and that, on rinsing off the deposited resin dispersion, there is a more or less severe dissolution of the deposited layer. No polyesterimides, polyamidimides or polyhydantoins are used.
In the German Auslegeschriften Nos. 2,248,836 and 2,842,626, processes are described for the electrophoretic deposition of cationic resins together with an additional resin from an aqueous dispersion, which lead to higher film thicknesses and show no metal dissolution. In these processes, pulverulent, nonionic synthetic resins are codispersed in a cationic binder and jointly deposited at the cathode. On the basis of their chemical structure, the resins, described in the Auslegeschriften, have no properties which would be suitable for coating wire. The preparation of a codispersed micronized powder is exceptionally expensive.